Field of the Invention
The present invention relates to an electrochromic display element, a display device, information equipment, and an electrochromic dimming lens.
Description of the Related Art
As for an electronic medium replacing paper, developments of electronic paper have been recently actively carried out. Examples of a display element used for the electronic paper include a display element using reflecting liquid crystals, a display element using electrophoresis, a display element using toner migration, and a display element using an electrochromic compound (an electrochromic display element).
The electrochromic display element is a strong candidate for a display element of the next generation, because it has a memory effect, and can be driven at low voltage. Therefore, a wide range of researches and developments have been currently conducted on the electrochromic display element, including a development of a material, and designing of a device.
An electrochromism phenomenon in the electrochromic display element is a reversible phenomenon that the electrochromic compound causes an oxidation/reduction reaction by applying voltage to the electrochromic compound to thereby color or discharge. The electrochromic display element can emit various colors with utilizing the electrochromism, as a result that a structure of the electrochromic compound is changed. Accordingly, there is a high expectation for the electrochromic display element as a display element capable of multicolor display.
As for the aforementioned electrochromic display element, for example, disclosed is an electrochromic element having an organic electrochromic layer, in which a plurality of polymer particle layers are laminated (see Japanese Patent Application Laid-Open (JP-A) No. 2003-121883). Moreover, disclosed is a multicolor display element containing a display layer, which is formed by laminating or mixing a plurality of electrochromic compositions each emitting different colors (see JP-A No. 2006-106669). Furthermore, disclosed is an electrochromic display device, in which a plurality of electrochromic layers corresponding to a plurality of display electrodes are provided (see JP-A No. 2010-33016).
Further, in order to realize practical electronic paper using an electrochromic display element, proposed is an electrochromic display device having excellent display quality without blurring in color and image blur, and having excellent durability, by using a thin film transistor (TFT) as a driving element, and forming a charge retention layer (high resistance layer) as a continuous layer (a solid film) on a pixel electrode (counter electrode) to which a plurality of the TFTs are formed with being apart from each other (see JP-A No. 2012-137736).
Moreover, the electrochromic display element has been considered to apply for a dimming lens, a dimming window, and a glare-proof mirror, other than the electronic paper.
The electrochromic display element used for electronic paper, or a dimming lens is desired to have excellent light fastness in a reflective display system and a transmissive display system. In addition, the electrochromic display element is desired to have the same degree of a white reflectance to that of paper in a reflective display system.
Conventionally, titanium oxide is suitably used in an electrochromic display element. Especially in an electrochromic display element of a reflective display system, titanium oxide particles having high white light scattering properties as a material of a white reflection layer, as well as using the titanium oxide particles as bearing particles of an electrochromic compound so that high white reflectance and high contrast ratio can be maintained.
However, titanium oxide, which is a photocatalystic active material, decomposes an electrochromic compound, and other constituting elements, to thereby reduce light fastness of an electrochromic display element. Use of rutile-type titanium oxide is proposed as the measure for preventing photocatalystic activities of the titanium oxide, but it is difficult to suppress reduction of the light fastness.
In order to inhibit photocatalystic activities of titanium oxide particles used as a material of a white reflection layer, moreover, disclosed is to cover surfaces of the titanium oxide particles with a polymer compound inert to a display medium composition (JP-A No. 2011-128200). However, the disclosed coating of the polymer compound cannot sufficiently suppress photocatalystic activities of the titanium oxide particles. Moreover, there is also a problem that no measure for suppressing the photocatalystic activities is provided on the titanium oxide particles used as bearing particles of the electrochromic compound.
Accordingly, there is a need for an electrochromic display element having excellent light fastness, and excellent coloring-discharging properties in a reflective display system and a transmissive display system, without increasing voltage required for coloring and discharging.